Certain types of fibre optic sensors employ a length of optic fibre arranged in such a way that a sensed parameter causes a strain to be imposed on the fibre. Typically the fibre is arranged in a coil, although other arrangements are possible. Such strain causes a change in phase of optical signal propagation in that fibre, which change can be detected by interferometric techniques. A variety of different arrangements for this type of transducer have previously been proposed, many of which have the coil of optic fibre wound on a deformable core or mandrel, which undergoes radial expansion or contraction in response to the sensed parameter, such as sensed vibration.
Such fibre optic sensors can exhibit extremely high sensitivities, and have the advantage of being completely passive, employing no power at the sensing transducer. Such sensors have also proved popular in applications where large arrays of sensors are required, on account of the relative ease with which they can be multiplexed.
An example of such an application is seismic surveying in the oil and gas exploration industry, where large time multiplexed arrays comprising hundreds or even thousands of vibration sensors and/or hydrophones can be used to sense reflections of an incident pulse from geological formations beneath the sea bed. Sampling such an array at regular periods provides 3D time lapsed data on existing or potential new reserves.
In greater detail, a high amplitude seismic source (usually an airgun) is towed across the top of a known or potential oilfield, firing the source at regular intervals, and the reflected returns form the source are monitored using sensors which are either towed together with the source or are positioned on the seabed. It is desired to be able to measure directly both the direct signal from the airgun when it first hits the sensors, and the seismic returns reflected from the underground features within the field, which have significantly lower amplitudes.
A problem experienced with this approach to sensing is that, for a given sampling rate, signals above a certain amplitude threshold cause the phase based sensed information to become distorted, and can cause failure of the demodulation process. This effect, commonly referred to as overloading or overscaling is dependent on the frequency of the measured signal. In seismic systems this can cause a particular problem with the direct arrival of the incident pulse, especially when that pulse has been generated close to the sensors (usually by an airgun towed from a surface vessel as it passes over the array). It is desirable to be able to record this incident pulse without the distortion that overscale can produce.
Applicant's co-pending International patent application No. PCT/GB2008/000830 describes apparatus and techniques for determining the derivative of the phase with respect to time which is imposed by a transducer (or a mulitiplexed array of transducers) on an interrogating signal. This technique is referred to as the derivative sensor technique (DST).
The rate of change, or derivative of the phase typically has a much smaller amplitude than the signal itself since the difference between the two times at which the signal is measured will usually be much less than the period of the signal being measured. Thus DST provides a reduced sensitivity measurement. For a signal with the majority of its energy centred at approximately 800 Hz, for example, the derivative of that signal will typically be attenuated by at least 60 dB with a period between the two measurement times of 200 ns.
PCT/GB2008/000830 describes multiple means of generating derivative signals with different amplitudes by using a different optical return methods and architecture, employing optical pulse pairs with different separations, where the length of separation determines the amplitude of the channel. This can result in derivative outputs with levels which are approximately 50 dB lower at 800 Hz (described as “medium DST”) and 38 dB lower at 800 Hz (described as “long DST”)
The level of the derivative signal is proportional to the difference in time between when the pulses pass through the sensor. Decreasing this time difference reduces the level of the derivative signal but increases the maximum level of the dynamic signal that can be measured. There is a practical limit, however, on the minimum time difference between pulse pairs in the multiplexed arrays described above.
It is an object of the present invention to provide improved sensing methods and apparatus, and an object of certain embodiments of the invention to provide improved methods and apparatus for sensing using a multiplexed fibre optic sensor array.